According to the SECAM standard, chrominance information in a video signal is transmitted by frequency modulation of the chrominance sub-carriers. The video signal comprises a sequence of lines forming a picture. Among these lines, two successive lines contain information corresponding to the red component and then information corresponding to the blue component of a three-color picture.
The sub-carriers on which the information for the red and blue are coded are centered on distinct frequencies, which are F.sub.OR =4.406 MHZ for the red and F.sub.OB =4.250 MHZ for the blue. Chrominance signal demodulators are used to extract information about the red and blue components from the signal, and more precisely from the frequency modulated sub-carriers.
FIG. 1 schematically illustrates the operating principle of a SECAM chrominance signal demodulator in a simplified manner. The demodulator in FIG. 1 can be used to demodulate lines containing information related to the red or blue. The demodulator includes an oscillator 10 with a controlled frequency and a phase comparator 20. For the purposes of this invention, a phase comparator means a device that has two signal inputs and is capable of outputting a current or a voltage proportional to a phase difference between the two input signals.
The oscillator 10 is a voltage controlled oscillator (VCO) comprising two inputs 12, 14 called the loop input and the adjustment input respectively, and an output 16 that outputs a signal at a frequency that depends on the voltages applied at the loop and adjustment inputs. The input to the loop 12 of oscillator 10 is connected to an output 26 of the phase comparator 20, while the output 16 from oscillator 10 is connected to a first input 22 of the phase comparator. A second input 24 of the phase comparator is connected to a two-way switch 28 that selectively connects the second input either to a chrominance channel 30 that transmits the chrominance signal, or to a quartz external reference oscillator 32 that outputs a signal at a reference frequency.
The phase comparator associated with the oscillator forms a PLL (Phase Locked Loop) demodulation loop. When the two-way switch 28 connects the second input 24 of the phase comparator to the chrominance channel 30, the demodulator outputs a voltage V proportional to the frequency of the chrominance signal F.
The voltage V is used as information representative of the color component (blue or red) corresponding to the signal present on channel 30. The voltage V is such that V=kFU, where k is a proportionality coefficient and U is a voltage applied to the adjustment input 14 of the oscillator. The frequency of the output signal from oscillator 10 must be determined very precisely and must remain stable to control the precision of the demodulator output voltage V so that the equipment having the demodulator will be capable of high fidelity reproduction of the colors.
Consequently, the oscillator 10 must be frequency matched to a reference frequency denoted F.sub.ref, supplied by the external oscillator 32. Oscillator 10 is matched by connecting the second input 24 of the phase comparator to the external reference oscillator 32. During the oscillator matching phase, an on-off switch 40 in an adjustment circuit 42 is closed. The matching phase may then be started, for example, by information that cancels out the video signal frame used to control the two-way switch 28 and the on-off switch 40.
This circuit comprises a transconductor 44, one input of which is connected to the input of oscillator loop 12, and the other input is connected to a reference voltage source 46 outputting a voltage V.sub.ref. When the voltage at the input to the oscillator loop 10 is not equal to the reference voltage V.sub.ref, the transconductor outputs a current (positive or negative) that charges or discharges a memory capacitor 48. Thus, the voltage U of the memory capacitor 48 applied to the oscillator adjustment input 14 is modified to adjust the oscillator output voltage to a value such that the oscillator loop input voltage becomes equal to V.sub.ref.
During a demodulation phase, the on-off switch 40 is open and the adjusted voltage U is maintained at the terminals of capacitor 48.
Thus V.sub.ref =kF.sub.ref U
let ##EQU1##
We can therefore write: ##EQU2## where H is the demodulation gain that is very precisely known as the ratio of the reference voltage to the reference frequency.
As already mentioned, the SECAM chrominance signal for successive lines of a picture is alternatively frequency modulated around the central frequencies with different values. A line received with central frequency FOR and a modulation factor corresponding to a first component of color (R-Y) is followed by a line with central frequency F.sub.OB, and a different modulation factor corresponding to another color component (B-Y). Thus a single demodulator like that described above is capable of demodulating the signal from two successive lines.
Therefore, a SECAM chrominance decoder must comprise two oscillators matched differently, as a function of two possible central frequencies F.sub.OB and F.sub.OR respectively, each having a phase comparator (the V.sub.ref and F.sub.ref generators may be common). Increasing the number of demodulators inhibits miniaturization requirements for SECAM signal decoders. Furthermore, the use of the two demodulators should be avoided because the memory capacitor is a component that occupies a very large area on an integrated circuit.